CYANIDE HEAP LEACHING

The successful application of heap leaching to the extraction
of gold from low-grade deposits has been one of
the main factors in higher output since the 1970s, especially in the
United States. It is a low cost process that extracts a soluble precious
metal or copper compound by dissolving the
metal content from the crushed ore.

Ore is heaped onto open-air leach pads with a base of asphalt or
impervious plastic sheeting. A sprinkler system is then laid along the
top of the ore pile through which a solution of dilute cyanide
is sprayed. The cyanide percolates down through
the heap for several weeks, leaching out the gold. This solution, now
enriched with gold, drains off the bottom of the pad
into what is known as the 'pregnant pond', from which it is pumped to
the recovery plant.

Cyanide has a natural affinity for gold , which
dissolves in it just as sugar would in a hot drink. Cyanidation has been
the principal method of extracting gold from ore since the development
of the MacArthur-Forrest Process in 1887,
which proved crucial in the development of the South African gold mining
industry.
The perfection of the cyanide process largely replaced amalgamation
with mercury that had previously been the
main method of extracting gold from ore. Cyanidation has also become
crucial since 1970 in gold recovery from low grade
deposits through heap
leaching . It should be noted that cyanide is
extremely toxic and must be handled with special care.

Heap leaching of gold was pioneered in the United States in 1973 at
Placer Development's Cortez open pit in Nevada
and proved on a larger scale at Pegasus Gold's Zortman Landusky mine in
Montana. Although it is low cost, recovery rates average only sixty to
seventy per cent, significantly less than with conventional milling.
But it has enabled low-grade ores, which
otherwise might not be economically viable, to be processed. In the
United States, where heap leaching is used most extensively, half of all
production is won by this method.